The present invention relates to photosensitive boranyl compounds and related compounds which generate active species (free radicals, organic bases, Lewis acids) when exposed to radiation.
Carbocyanine alkyltriphenylborate salts have proven effective visible light sensitive photoinitiators for rapidly polymerizing polyolacrylate monomer systems as described in U.S. Pat. Nos. 4,772,541 and 4,772,530 to Gottschalk, et al.; U.S. Pat. No. 4,859,572 to Farid, et al.; U.S. Pat. No. 4,847,236 to Satomura, et al.; U.S. Pat. No. 4,902,604 to Yamaguchi, et al.; U.S. Pat. No. 4,950,581 to Koike, et al; U.S. Pat. No. 4,971,891 to Kawamura, et al.; and U.S. Pat. No. 5,055,372 to Shanklin, et al.
It has been shown that the photoreactions of carbocyanine alkyltriphenylborate salts involve an electron transfer from the borate to the singlet state of the cyanine cation by Chatterjee, et al., in J. Am. Chem. Soc. 1988,110,2326 and J. Am. Chem. Soc. 1990,112,6329. Because the lifetime of the cyanine singlet is short (pico seconds), the electron transfer is observed only in nonpolar solvents where the transfer can occur from the tight ion pair. It has been reported by Hassoon et al., J. Phys. chem. 1995, 99, 9473 that photoreduction of fluorone dyes by triphenylbutyl borate salts is useful for the initiation of free radical polymerization, and that intermolecular electron transfer from the borate anion to the triplet state of the dye leads to the formation of butyl radicals which initiate polymerization.
As disclosed by Farid et al., in U.S. Pat. No. 4,959,572, activators are recognized in the art to fall into two distinct classes. One class of activators is referred to as electron acceptor activators. These activators liberate a free radical capable of initiating ethylenic addition by accepting an electron from a photosensitizer in its excited state. The reactions can be diagramed as follows: ##STR1## where Sens represents a photosensitizer,
.lambda. represents exposure to actinic radiation, PA1 * indicates the excited state produced by light absorption on exposure, PA1 A.sup.+ -R represents the electron acceptor activator, PA1 A.sup.+ represents the electron accepting moiety of the activator before an electron is accepted, PA1 A represents the electron accepting moiety of the activator after an electron is accepted, PA1 R represents the moiety that is ultimately cleaved as a free radical, PA1 D.sup.- represents the electron donating moiety of the activator before donating an electron, PA1 D represents the electron donating moiety of the activator after donating an electron,
Sens.sup.+.cndot. shows the photosensitizer converted to a cation radical by loss of an electron, and .cndot. denotes a radical.
Another class of activators is referred to as electron donor activators. These activators are oxidized by donating an electron to the excited state of the photosensitizer, thus liberating a free radical capable of initiating ethylenic addition. The reactions can be diagrammed as follows: ##STR2## where D.sup.- -R' represents the electron donor activator,
Sens.sup.- shows the photosensitizer converted to an anion by loss of an electron, and the retaining symbols are as indicated above.
A survey of electron acceptor and electron donor activators useful with photosensitizers is set forth in Volman et al., Advances in Photochemistry, Vol. 13, in the chapter titled "Dye Sensitized Photopolymerization" by D. F. Eaton, pp. 427 to 488, John Wiley & Sons (1986).
Specht and Farid in published U.K. Specification No. 3,083,832A disclose photopolymerization coinitiators including an azinium electron acceptor activator and acting as a photosensitizer, an amino-substituted ketocoumarin.
Gottschalk et al in published European Patent Application No. 0223 587 disclose a polymerization system including as an initiator a zwitterion consisting of an undissociated borate anion and a dye cation. Disclosed cationic dyes include Methylene Blue, Safranine O, Malachite Green, and various cyanine and rhodamine dyes. For example, U.S. Pat. Nos. 4,772,541 4,772,530 and 5,055,372 all to the Mead Corporation; U.S. Pat No. 4,859,572 to Eastman Kodak corporation; and U.S. Pat. Nos. 4,859,236, 4,902,604 and 4,950,581 to Fuji Photo Film Co., Ltd. teach that carbocyanine alkyltriphenylborate salts have proven to be effective visible light sensitive photoinitiators which rapidly polymerize polyolacrylate monomer systems.
European Patent Application No. O 690 074 A1 to Toyo Ink Manufacturing Co., Ltd. published Jan. 3, 1996 discloses a photopolymerization initiator composition containing (A) a sulfonium organoboron complex or oxosulfonium organoboron complex; and (B) a sensitizer which has electron donating capability and electron acceptability in an excited state. Aromatic ketones not linked to the borate via a cationic linking group are disclosed as being sensitizers. Japanese Kokai Tokyo Koho JP 08 03210 discloses organosulfonium borates as initiators for polymerization of unsaturated compounds upon irradiation by light or heating.